I think most of the problems with Mach4 have been dealt with, but I think there's still a few key features that don't have good or any kind of interface yet.

From the latest discussions I've seen, I'd sum it up by saying it's probably worth it over Mach 3, however there simply isn't the same knowledge base, or examples for Mach 4. I know the default screenset is currently being re-worked.

There is nothing stopping you from downloading Mach 4 and trying it out. That way you'll get a far better idea rather than relying on opinions of people who're only speculating. I'll admit I'm also just speculating, as I've not tried Mach4 since one of the first public releases, and am unlikely to use Mach4 as I'm moving to KMotionCNC.